The Kushans possibly used the Greek language initially for administrative purposes, but soon began to use the Bactrian language. Kanishka sent his armies north of the Karakoram Mountains. A direct road from Gandhara to China remained under Kushan control for more than a century, encouraging travel across the Karakoram and facilitating the spread of Mahayana Buddhism to China. The Kushan dynasty had diplomatic contacts with the Roman Empire, Sasanian Persia, the Aksumite Empire and the Han dynasty of China. The Kushan Empire was at the center of trade relations between the Roman Empire and China: according to Alain, "for a time, the Kushana Empire was the center point of the major civilizations". While much philosophy, art, and science was created within its borders, the only textual record of the empire's history today comes from inscriptions and accounts in other languages, particularly Chinese. The Kushan Empire fragmented into semi-independent kingdoms in the 3rd century AD, which fell to the Sasanians invading from the west, establishing the Kushano-Sasanian Kingdom in the areas of Sogdiana, Bactria and Gandhara. In the 4th century, the Guptas, an Indian dynasty also pressed from the east. The last of the Kushan and Kushano-Sasanian kingdoms were eventually overwhelmed by invaders from the north, known as the Kidarites, and then the Hephthalites.